Technical Field
Embodiments of the subject matter disclosed herein generally relate to methods and systems and, more particularly, to mechanisms and techniques for determining a field trajectory for a land-based seismic survey to improve 4-dimensional (4D) repeatability.
Discussion of the Background
During the past years, interest in monitoring oil and/or gas reserves has increased. Time-lapse (or 4D) seismic monitoring of producing oil fields is an accepted optimization method for field development and product recovery, providing significant recovery rate improvements and drilling cost savings.
Time-lapse seismic reservoir monitoring is the comparison of 3D seismic surveys at two or more points in time. Time-lapse seismic reservoir monitoring also has potential for increasing ability to image fluid movement between wells. A traditional configuration for achieving a 4D land seismic monitoring is illustrated in FIG. 1, which shows a system 10 that includes plural receivers 12 positioned over an area 12a of a subsurface to be explored and in contact with the surface 14 of the ground. A number of vibroseismic sources 16 (e.g., located on corresponding trucks) are also placed on the surface 14 in area 16a, in the vicinity of area 12a of the receivers 12. A recording device 18 is connected to the plurality of receivers 12 and placed, for example, in a station truck 20. Each source 16 may be composed of a variable number of vibrators, typically between one and five, and may include a local controller 22. A central controller 24 may be present to coordinate the sources' 16 shooting times. A global positioning system (GPS) 26 may be used to time-correlate the sources 16 and receivers 12.
With this configuration, sources 16 are controlled to generate seismic waves, and the plurality of receivers 12 record waves reflected by the oil and/or gas reservoirs and other structures. The seismic survey may be repeated at various time intervals, e.g., months or years apart, to determine changes in the reservoir. For reservoir monitoring, it is traditional to maintain the receivers at their locations in the field over the entire time of the 4D surveys (i.e., not to remove the receivers at the end of a first survey and to deploy them again at the beginning of a second survey). It is also customary to have mobile sources that move from location to location and shoot seismic waves. For this case, when the first survey ends, the sources are removed, and when the second survey starts, the same sources or other sources are brought back in.
For this situation, it is desirable to position and shoot the sources at the same geographic positions during each survey, i.e., the first survey, the second survey, etc., of the 4D survey. However, the following problems are noted in practical situations. FIG. 2 shows a system 200 that includes plural sources 202 and plural receivers 204. A single source 202 is shown for simplicity. The receivers are fixed, i.e., their locations do not change during the 4D survey. However, the sources are truck-mounted and they carried from one shooting position to another by truck. This means that the truck driver 206 is instructed to follow a given path 210 each time an area 211 is surveyed. Path 210 is traditionally pre-calculated by the operator of the seismic survey, usually at its facilities, which can be hundreds, if not thousands, of miles from the surveyed area 211.
Because the operator relies on maps for determining path 210, which may not be accurate or omit certain details that influence the path actually followed by the driver, path 210 can intersect with an obstacle or avoidance 214 (e.g., a hill, a pond, a man-made structure, a boulder, etc.). In one case, the given path does not match the truck specifications (e.g., steering angle too wide, steep slope which the truck cannot climb, etc.). Consequently, the driver cannot follow given path 210 and cannot shoot source 202 at the required locations 210A, 210B, etc. In these situations, the driver takes the liberty of deviating from given path 210 and following another path 216, which the driver believes to be close enough to given path 210. This means that source 202 is shot at locations 216A, 216B, etc. However, the driver's choice may change from survey to survey, thus, introducing undesirable inaccuracies in the collected seismic data (geographic discrepancies 220 between the intended shooting positions and the positions actually shot, which vary along the path).
Given the fact that operators try to understand the behavior of the reservoir over time by qualitatively and quantitatively quantifying these effects, 4D reservoir monitoring is very sensitive to shooting sources at the same locations when the survey is repeated.
Thus, there is a need for developing a device and a method for minimizing geographic discrepancies between shoots intended to be performed at the same position over time.